The preparation of powder coatings is mostly carried out by extruding the powder coating ready formulated by dry mixing of all the required components (powder coating fresh material) in the form of a paste-like melt, cooling off of the melt, coarse comminution into so-called powder coating chips, fine milling (grinding) and subsequent sieving and classifying to desired grain fineness (powder coating classification). Powder coating material that is too coarse-grained (high grain) on sieving can again be fine-milled. Powder coating material that is too fine-grained (low grain) represents waste.
Powder coating waste also arises in the form of powder coating over-spray during the powder coating application.
Powder coating waste should if possible be recovered as powder coating to be employed for the original use, not only because of environmental considerations, but also because powder coatings are valuable materials.
Various methods that allow recycling of powder coating waste to be used for the original purpose are described in the patent literature.
DE-A40 28 567 describes the direct recycling of powder coating over-spray by addition of the over-spray to fresh material before or during extrusion, thus making the process complicated to handle.
In WO 96/15891 pressing of powder coating waste into friable tablets, which are then extruded together with fresh powder coating material, is described.
It is known from EP-A-0-683 199 to form a layer of powder coating waste, to expose the latter to heat, wherein the powder coating particles coalesce without degradation or cross-linking of the powder coating. IR-radiators can serve as the heat source. The coalesced powder coating particles are then again extruded alone or together with powder coating fresh material.
The processes known from DE-A-40 28 567, WO 96/15891 and EP-A-0-683 199 include a re-extrusion of the powder coating waste. This is linked to the danger of a too high thermal load.
WO 98/33848 describes the agglomeration of finely divided powder coating residues under the influence of pressure. The agglomerates can be returned to the powder coating preparation process.
WO 99/23068 and WO 99/23176 describe compacting of powder coating waste by applying pressure. The compacted powder coating waste can be ground together with fresh powder coating material that is to be ground.
The powder coating compacts prepared according to the processes known from WO 98/33848, WO 99/23068 and WO 99/23176 are only restrictively suitable for the preparation therefrom, by grinding, of powder coatings having the desired particle size distribution.
U.S. Pat. No. 6,162,861 describes the preparation of powder coating particles having a particle size suited for application by agglomeration of powder coating fine grain by heat and preferably with the use of auxiliary products promoting the agglomeration. The process is extremely difficult to control due to the strict requirements concerning the particle size distribution of the powder coating suited for application.
There is a requirement for providing an effective process for recycling powder coating waste. The powder coating material to be recycled should thereby be subjected as little as possible to heat. Any change in composition of the powder coating should be avoided. Likewise the addition for example of auxiliary products should be avoided.